U.S. patent application number 10/528386 was filed with the patent office on 2006-07-13 for beam plasma source.
Invention is credited to John E. Madocks.
Application Number | 20060152162 10/528386 |
Document ID | / |
Family ID | 32030790 |
Filed Date | 2006-07-13 |
United States Patent
Application |
20060152162 |
Kind Code |
A1 |
Madocks; John E. |
July 13, 2006 |
Beam plasma source
Abstract
A plasma source which includes a discharge cavity having a first
width, where that discharge cavity includes a top portion, a wall
portion, and a nozzle disposed on the top portion and extending
outwardly therefrom, where the nozzle is formed to include an
aperture extending through the top portion and into the discharge
cavity, wherein the aperture has a second width, where the second
width is less than the first width. The plasma source further
includes a power supply, a conduit disposed in the discharge cavity
for introducing an ionizable gas therein, and at least one cathode
electrode connected to the power supply, where that cathode
electrode is capable of supporting at least one magnetron discharge
region within the discharge cavity. The plasma source further
includes a plurality of magnets disposed adjacent the wall portion,
where that plurality of magnets create a null magnetic field point
within the discharge cavity.
Inventors: |
Madocks; John E.; (Tucson,
AZ) |
Correspondence
Address: |
WILLIAM C. CAHILL
155 PARK ONE
2141 E. HIGHLAND AVENUE
PHOENIX
AZ
85016
US
|
Family ID: |
32030790 |
Appl. No.: |
10/528386 |
Filed: |
September 19, 2003 |
PCT Filed: |
September 19, 2003 |
PCT NO: |
PCT/US03/29204 |
371 Date: |
December 23, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60412051 |
Sep 19, 2002 |
|
|
|
Current U.S.
Class: |
315/111.21 |
Current CPC
Class: |
H05H 1/46 20130101; H01J
2237/0815 20130101; H05H 1/48 20130101; H01J 37/32357 20130101;
H01J 37/3266 20130101; H01J 37/32009 20130101; H01J 27/146
20130101 |
Class at
Publication: |
315/111.21 |
International
Class: |
H01J 7/24 20060101
H01J007/24 |
Claims
1. A plasma source, comprising: a discharge cavity having a first
width, wherein said discharge cavity includes a top portion and a
wall portion. a nozzle disposed on said top portion and extending
outwardly therefrom, wherein said nozzle is formed to include an
aperture extending through said top portion and into said discharge
cavity, wherein said aperture has a second width, wherein said
second width is less than said first width; at least one cathode
electrode connected to said first power supply, wherein said
cathode electrode is capable of supporting at least one magnetron
discharge region within said discharge cavity; a plurality of
magnets disposed adjacent said wall portion, where in said
plurality of magnets create a null magnetic field point within said
discharge cavity; a conduit, other than said aperture, disposed in
said discharge cavity for introducing an ionizable gas into said
discharge cavity.
2. The plasma source of claim 1, wherein said ionizable gas is
injected between said cathode and said nozzle within said discharge
cavity.
3. The plasma source of claim 1, wherein said plurality of magnets
comprises one or more electromagnets.
4. The plasma source of claim 1, wherein two of the three axial
magnetic field regions adjacent to said null point pass through
said cathode surface, and wherein the third axial magnetic field
comprises the mirror confinement region emanating through said
nozzle.
5. The plasma source of claim 1, wherein said null magnetic field
point is located along the center-line of said aperture.
6. The plasma source of claim 1 wherein said cathode material
comprises a secondary electron emission coefficient greater than
about 1.
7. The plasma source of claim 1, wherein said nozzle is
interconnected with said first power supply such that said nozzle
comprises an anode.
8. The plasma source of claim 1, wherein said nozzle is
electrically floating.
9. The plasma source of claim 1, wherein said nozzle is
electrically connected to ground.
10. The plasma source of claim 1, further comprising a second power
supply, wherein said second power supply is connected to said
nozzle such that said nozzle comprises an anode.
11. The plasma source of claim 7, wherein said second power supply
is selected from the group consisting of a DC power supply, an AC
power supply, and RF power supply.
12. A plasma processing apparatus, comprising: a beam plasma source
comprising a discharge cavity having a first width, wherein said
discharge cavity includes a top portion and a wall portion; a
nozzle disposed on said top portion and extending outwardly
therefrom, wherein said nozzle is formed to include an aperture
extending through said top portion and into said discharge cavity,
wherein said aperture has a second width, wherein said second width
is less than said first width; a power supply, wherein said wall
portion is interconnected to said power supply and wherein said
wall portion comprises a cathode; a plurality of magnets disposed
adjacent to and external to said discharge cavity, wherein said
plurality of magnets create a null magnetic field point within said
discharge cavity; a conduit, other than said aperture, disposed in
said discharge cavity for introducing an ionizable gas into said
discharge cavity; a process chamber, wherein said beam plasma
source is disposed within said process chamber; a substrate
disposed within said process chamber, wherein said substrate is
external to said beam plasma source.
13. The plasma processing apparatus of claim 12, further comprising
an anode disposed within said process chamber, wherein said anode
is not physically attached to said plasma beam source.
14. The plasma processing apparatus of claim 12, wherein said beam
plasma source further comprises a cusp magnetic field producing at
least one magnetron confinement one within said cathode cavity.
15. A plasma processing apparatus, comprising: an enclosure
defining a cavity, wherein said enclosure is formed to include a
nozzle; a power supply interconnected with said enclosure such that
said enclosure comprises a cathode electrode; a cusp magnetic field
defining a null magnetic field point disposed within said cavity;
wherein said cusp magnetic field comprises a first portion and a
second portion, wherein said first portion creates a closed drift
electron magnetron confinement region within said cathode cavity,
and wherein said second portion produces a mirror confinement
region passing through said nozzle.
16. A method to treat a substrate with a plasma beam, comprising
the steps of: providing a beam plasma source comprising a discharge
cavity having a first width, wherein said discharge cavity includes
a top portion and a wall portion; a nozzle disposed on said top
portion and extending outwardly therefrom, wherein said nozzle is
formed to include an aperture extending through said top portion
and into said discharge cavity, wherein said aperture has a second
width, wherein said second width is less than said first width; a
power supply, wherein said wall portion is interconnected to said
power supply and wherein said wall portion comprises a cathode; a
plurality of magnets disposed adjacent to and external to said
discharge cavity; a conduit, other than said aperture, disposed in
said discharge cavity for introducing an ionizable gas into said
discharge cavity; providing a process chamber; disposing said beam
plasma source within said process chamber; providing a substrate;
disposing said substrate within said process chamber, wherein said
substrate is external to said beam plasma source; creating null
magnetic field point within said discharge cavity; introducing an
ionizable gas into said discharge cavity via said conduit; igniting
a plasma within said discharge cavity; projecting said plasma
through said nozzle; directing said plasma onto said substrate.
17. The method of claim 16, further comprising the steps of:
generating a plurality of electrons within said discharge cavity,
wherein a portion of said plurality of electrons passing through
said null magnetic field point pass out of said discharge cavity
through said nozzle.
18. The method of claim 17, further comprising the step of forming
three mirror magnetic field electron confinement zones within said
discharge cavity, wherein each of said three mirror magnetic field
electron confinement zones extend outwardly from said null magnetic
field point.
19. The method of claim 18, wherein one of said mirror magnetic
field electron confinement zones extends through said aperture.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to plasma and ion sources used
for industrial processes such as plasma treatment, plasma enhanced
chemical vapor deposition (PECVD) and plasma etching and to
electric propulsion devices for space applications.
BACKGROUND OF THE INVENTION
[0002] Plasma and ion sources are usefully applied in a number of
processes including: Plasma enhanced chemical vapor deposition
(PECVD), reactive ion etching, plasma surface modification and
cleaning, increasing the density of evaporated or sputtered films
and assisting a reactive evaporation or sputtering process. Of
growing interest is the application of these processes to larger
substrates such as flexible webs, plasma televisions and
architectural glass.
[0003] Several plasma and ion sources are commercially available
and many more have been disclosed. Commercially available plasma
and ion sources include: Hollow cathode plasma sources, gridded ion
sources, end hall ion sources, closed drift type ion sources
including extended acceleration channel and anode layer types, and
impeded anode types like the Leybold Optics' Advanced Plasma
Source. While successfully applied to small substrate applications
like semiconductors or optical filters, they are less effective in
processing wide substrate applications. This is primarily due to
the use of point electron sources for beam creation and
neutralization rather than uniform, linear electron sources.
[0004] Point electron source technologies such as filaments, heated
low work function materials and hollow cathodes are difficult to
extend linearly. Consequently, the ion and plasma sources that rely
on these point electron sources have difficulty producing the
uniform linear beams when utilizing large area substrates.
[0005] Therefore, there is a need for a uniform, linear plasma or
ion source that can be readily extended to wide substrates. This
ideal linear source should not require a delicate or expensive
electron source, such as filaments or LaB6, and should be capable
of operating over a wide process pressure range. This source should
also be physically compact, economical, and should produce a dense,
efficient plasma beam.
[0006] Prior art sources generally utilize one of two technology
categories. One such category comprises magnetron sputtering
sources, and more specifically unbalanced magnetrons and hollow
cathode sputtering sources. The second such category comprises
plasma and ion sources.
Unbalanced Magnetron Sources
[0007] Window and Savvides presented the concept of unbalanced
magnetron ("UBM") sputter cathodes in several published articles.
In these articles, the Type II unbalanced magnetron is disclosed
with its ability to ionized the sputtered flux from the cathode.
The fundamental operating principles of the null magnetic field
region and mirror magnetic confinement electron trapping are
taught.
[0008] FIG. 12 shows a planar target type II UBM as presented by
Window and Savvides. Window and Harding later disclosed a type II
UBM without a central magnetic material or high permeability pole.
In FIG. 12, magnets 200 are configured around the periphery of a
rectangular or round shunt plate 201. Central soft iron pole 207 is
located in the center of the shunt plate Because of the
`unbalanced` nature of the magnetic arrangement, a null field point
203 is created above magnetron trap 205 and strengthening field
lines above the null point produce a mirror confinement region 208.
In operation, magnetron plasma 204 sputters the target 206.
Electrons leaving the magnetron plasma are trapped in the mirror
containment region 208 creating a second visible plasma region.
[0009] As presented in the literature, the mirror plasma region
ionizes a significant portion of the sputter flux from the target.
The plasma 208 generated in the mirror region also projects out to
the substrate 209 and usefully bombards the growing sputtered film.
Plasma 208 can be used for plasma processes such as PECVD, plasma
treatment etc. While finding use in these plasma processes, the
sputtered flux from the target 206 is not always welcome, the UBM
must operate in the mTorr range typical for magnetron sputtering
and, for PECVD applications, the exposed target 206 is quickly
contaminated by condensing PECVD constituents.
Hollow Cathode Sputter Sources
[0010] The term Hollow Cathode has been used to describe a variety
of sputter sources in the prior art. U.S. Pat. No. 4,915,805
discloses a hollow cathode confined magnetron with the substrate
passing through the center of the cavity. U.S. Pat. No. 4,933,057
discloses a hollow cathode configured magnetron with an anode
positioned opposite from the opening into the process chamber. The
anode in this position will allow electrons to reach the anode
without having to pass out of the discharge cavity first. No gas is
introduced into the discharge cavity separate from the opening to
the process chamber.
[0011] U.S. Pat. No. 5,073,245 teaches a sputter source in a cavity
separate from the process chamber. The magnetic field is along the
axis of the cavity cylinder and a magnetron type containment region
is reported to be created around the inside of the cavity cylinder
walls. The opening to the process chamber creates a discontinuity
in the magnetron racetrack. Anodes are located inside the cavity,
at each end. U.S. Pat. No. 5,334,302 discloses a sputtering
apparatus comprising multiple magnetron cathode cavities. Process
gas is introduced into the base of each cavity. The cavities are
open to the process chamber.
[0012] U.S. Pat. No. 5,482,611 discloses an unbalanced magnetron
sputter cathode with a cup-shaped or annular cathode. A null
magnetic field point is produced adjacent to the cathode opening.
The discharge cavity is open to the process chamber. In FIG. 6 of
the '611 patent a separate microwave applicator is fitted over the
cathode opening. Though separate from the cathode, the applicator
opening dimensions are equal to or larger than the cathode cavity.
In one embodiment process gas is introduced into the cavity at the
base of the cavity opposite the process chamber opening.
[0013] U.S. Pat. No. 5,908,602 teaches a linear arc discharge
source. The discharge cavity does not include a magnetron confined
plasma region and the discharge cavity opening is exposed to the
process chamber.
[0014] U.S. Pat. No. 6,444,100 discloses a box shaped hollow
cathode sputter source. The bottom of said box is either
electrically floating or connected to the cathode. The box is open
to the process chamber and process gas is not introduced into the
box other than via the process chamber opening.
Other Plasma Sources
[0015] U.S. Pat. No. 6,444,945 teaches a bipolar plasma source,
plasma sheet source, and effusion cell utilizing a bipolar plasma
source. In the preferred embodiments a magnetron cathode plasma is
not created and the hollow cathode cavity opening is exposed to the
process chamber. U.S. Pat. No. 4,871,918 discloses a hollow-anode
ion-electron source comprising a discharge cavity with a reduced
dimension opening conduit to the process chamber. There is no
magnetron confined region or null magnetic field point within the
discharge cavity.
[0016] U.S. Pat. No. 6,103,074 teaches a cathode arc vapor
deposition method and apparatus that implements a cusp magnet
field. There is no magnetron confined region inside the discharge
cavity and the cavity is open to the process chamber.
SUMMARY OF THE INVENTION
[0017] Applicant's invention includes a plasma source. Applicant's
plasma source includes a discharge cavity having a first width,
where that discharge cavity includes a top portion, a wall portion,
and a nozzle disposed on the top portion and extending outwardly
therefrom, where the nozzle is formed to include an aperture
extending through the top portion and into the discharge cavity,
wherein the aperture has a second width, where the second width is
less than said the width.
[0018] Applicant's plasma source further includes a power supply, a
conduit disposed in said discharge cavity for introducing an
ionizable gas into the discharge cavity, and at least one cathode
electrode connected to the power supply, where that cathode
electrode is capable of supporting at least one magnetron discharge
region within the discharge cavity. Applicant's plasma source
further includes a plurality of magnets disposed adjacent the wall
portion, where that plurality of magnets create a null magnetic
field point within the discharge cavity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 shows a section view of Applicant's beam source;
[0020] FIG. 2 shows a top section view of the apparatus of FIG.
1;
[0021] FIG. 3 shows an isometric view of the apparatus of FIG.
1
[0022] FIG. 4 shows a view of Applicant's beam source comprising
separate gas inlets, with the beam directed toward a substrate
and;
[0023] FIG. 5 shows a view of Applicant's beam source used to
assist reactive deposition in an electron beam evaporation
application;
[0024] FIG. 6 shows a side view of Applicant's beam source applied
to a planetary/box coating application;
[0025] FIG; 7 shows Applicant's beam source with the plasma
directed onto a translating, biased substrate;
[0026] FIG. 8 shows two beam sources facing each other with
opposite pole magnets;
[0027] FIG. 9 shows a section view of an electromagnet version of
Applicant's invention for a space thruster application;
[0028] FIG. 10 shows an embodiment comprising an electrical power
arrangement enhancing the ion source aspects of the present
invention;
[0029] FIG. 11 shows a section view of Applicant's beam source
implementing vertically oriented magnets and a planar cathode;
[0030] FIG. 12 shows a section view of a prior art unbalanced
magnetron sputter source.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED
[0031] FIG. 1 shows a section view of beam source 24 producing a
beam of dense plasma 9 projecting outwardly from nozzle 6. Aperture
101 extends through nozzle 6, into discharge cavity 26. Discharge
cavity 26 has a first width 110. Aperture 101 has a second width
115, where the second width 115 is less than the first width 110.
Center-line 120 comprises the middle of first width 110.
[0032] In certain embodiments, discharge cavity 26 comprises a
parallelepiped having a rectangular cross section. In these
embodiments, the first width 110 comprises the length of the longer
side of that rectangular cross section. In certain embodiments,
discharge cavity 26 a parallelepiped having a square cross section.
In these embodiments, the first width 110 comprises the length of
one side of that square cross section. In certain embodiments,
discharge cavity 26 comprises a cylinder having a circular cross
section. In these embodiments, the first width 110 comprises the
diameter of that circular cross section.
[0033] In certain embodiments, aperture 101 has a rectangular cross
section. In these embodiments, second width 115 comprises the
length of the longer side of that rectangular cross section. In
certain embodiments, aperture 101 has a square cross section. In
these embodiments, second width 115 comprises the length of one
side of that square cross section. In certain embodiments, aperture
101 has a circular cross section. In these embodiments, second
width 115 comprises the diameter of that circular cross
section.
[0034] Source 24 is disposed within a process chamber, not shown,
where that process chamber is maintained at a reduced pressure.
Magnets 1 and 2 are disposed facing each other with the south poles
supported by mild steel shunt 3. By "facing each other," Applicant
means that the pole of magnet 1 having a first magnetic polarity
has a facing relationship with the pole of magnet 2 having that
same magnetic polarity. The magnets 1 and 2 produce a cusp magnetic
field composed of outwardly directed field lines 18 and inwardly
directed lines 19. The inward lines 19 pass through insulator 15
and liner 16 to center shunt 10. The cusp magnetic field creates a
null magnetic field point 25 inside discharge cavity 26. In certain
embodiments, null magnetic field point 25 is located along
center-line 120. Along with end magnets not shown and magnets 1 and
2, the cusp fields 18 and 19 create endless electron traps in
regions 9 and 8. Shunt 10 is connected to shunt 11, and both are
made of mild steel. Liner 16 is brazed to block 12 to improve heat
transfer.
[0035] Block 12 is water cooled via holes 13 in combination with
associated piping not shown. Shunt 11 is fastened to block 12. The
assembly of the liner 16, block 12 and shunts 10 and 11 form one
electrode of the source. The second electrode is formed by shunt
box 3 and cover 5. The magnets are ceramic type isolated from liner
16 and block 12 by insulators 14 and 15. In certain embodiments,
insulators 14 and 15 are formed from one or more fluoropolymers. In
other embodiments, insulators 14 and 15 are formed from an
electrically insulating ceramic material.
[0036] Gap 100 separates separate box 3 from block 12 and shunt 11
to eliminate plasma in the gap. In certain embodiments, gap 100 is
about 3 mm. Gas 27 is introduced into the source through port 4 in
box 3. The gas 27 travels around block 12 via gap 100 between box
and block 12; Gas 27 then flows through a plurality of grooves 22
disposed in box 3 and cover 5. Gas 27 is introduced into discharge
cavity 26 between cover 5 and liner 16. Cover 5 includes a nozzle 6
though which the gas 27 flows into the process chamber. The cover 5
and nozzle 6 are water cooled with brazed-on tubing 7. One side of
power supply 17 is connected to cover 5, box 3, and to chamber
ground.
[0037] The other pole of power supply 17 is connected to internal
block assembly 12, and thereby liner 16 and shunts 10 and 11. The
electrical connection to block 12 is made to the water cooling
tubing exiting box 3 (tubing not shown). In certain embodiments,
liner 16 comprises a cathode electrode. In certain embodiments,
liner 16 is formed from materials having a secondary electron
emission coefficient .delta. of about 1 or more.
[0038] In certain embodiments, power supply 17 comprises a standard
sputter magnetron type. In certain embodiments, power supply 17
comprises a pulsed DC supply. In certain embodiments, power supply
17 comprises a mid-frequency AC supply. In certain embodiments,
power supply 17 comprises an RF supply.
[0039] In the illustrated embodiment of FIG. 1, a DC supply 17 is
used with the negative electrode connected to block 12. When gas 27
is introduced into discharge cavity 26 and power supply 17 is
turned on, a plasma is ignited in regions 8 and 9 of the source.
Region 8 is an endless Hall current contained plasma extending the
length of the source. The two lobes of region 8, as seen in section
view FIG. 1, appear as an extended donut of plasma when the inside
of the operating source is viewed. This region 8 is created when
the electric field from cover 5 penetrates down past magnetic field
lines 19 inside the source. As electrons attempt to follow these
electric field lines they are restricted by magnetic field lines
19.
[0040] Electrons cannot escape from the electrostatically and
magnetically confined region made by electron containing liner 16
and shunt 10 surfaces and field lines 19. The result is a confined
plasma region 8 inside discharge cavity 26. Region 9 is created and
sustained as a result of plasma 8. By the arrangement of magnetic
field lines 18, cover 5 and nozzle 6, electrons created by plasma 8
are prevented from reaching the cover 5 and nozzle 6 anode
electrode. Field lines 18 pass outwardly from liner 16, converge,
and pass outwardly through nozzle 6.
[0041] Because electrons cannot cross magnetic field lines, the
electric circuit between cover 5, nozzle 6 and plasma 8 can only be
completed by the electrons exiting through nozzle 6 and passing out
of the magnetic field 18 containment region. Plasma 9 is created
because, when electrons attempt to escape along magnetic field
lines 18 through the nozzle 6, they are confronted with a magnetic
mirror as field lines 18 converge in nozzle 6. This mirror region
reflects a portion of the electrons and creates a second
containment region 39 within plasma 9.
[0042] Region 39 is again a closed drift magnetic bottle as
electrons move in a cyclodial motion down to one end of the source
and back to the other. In addition to the electron escape path,
nozzle 6 also forms the only escape path for gas 27 flowing from
discharge cavity 26 into the process chamber. The process gas 27 is
forced through plasma region 39 where a high percentage of gas 27
is ionized prior to exiting nozzle 6. The confluence of gas 27 and
electrons in region 39 creates a dense plasma 9 that extends
outwardly from nozzle 6 into the process chamber. When the source
24 is viewed in operation, it appears that plasma 39 and plasma 9
comprise a single plasma. The internal diameter of nozzle 6 is
smaller than the internal diameter of discharge cavity 26. By
making nozzle 6 narrower, not only is less sputtered material from
liner 16 able to reach the process chamber, but the process gas 27
must pass through plasma region 39 to exit discharge cavity 26.
[0043] FIG. 2 shows a top view beam source 24 with cover 5 removed.
End magnets 20 and 21, in combination with side magnets 1 and 2,
create the closed drift magnetic fields 18 and 19, with only
magnetic field 18 shown in FIG. 2. FIG. 2 also includes box 3,
liner 16, insulator 15 and, below magnets 1, 2, 20 and 21, water
cooled block 12. Plurality of grooves 22 in box 3 for gas 27 are
also illustrated. Plasma 9 is shown as the darker portion in the
center. The lighter portion corresponds to plasma region 39.
[0044] FIG. 3 shows an isometric view of beam source 24 where
certain water cooling piping is not shown. As described above, this
water piping is useful to make electrical connections to both
electrodes. In the illustrated embodiment of FIG. 3, plasma 9
emanates outwardly from nozzle 6 into the process chamber. As
shown, plasma 9 forms a narrow uniform beam extending outwardly
from nozzle 6.
[0045] As those skilled in the art will appreciate, beam source 24
may comprise many shapes, sizes, scales, and may include a
plurality of materials. For example, in one embodiment source 24
was constructed as follows: Magnets 1 and 2 were ceramic type
measuring about 1'' wide.times.about 4'' long.times.about 1''
thick. Magnets 20 and 21 were about 1'' wide.times.about 2''
long.times.about 1'' thick. Block 12 was formed from brass. Top
cover 5 and nozzle 6 were formed of copper. The opening in nozzle 6
was about 0.75'' wide.times.about 0.75'' deep.times.about 3.5''
long. Shunt 10 and shunt 11 were formed of mild steel. Liner 16 was
formed of copper sheet bent into an oval shape, with the long
internal diameter of that oval measuring about 1.5''. As those
skilled in the art will appreciate, many variations and
modifications can be made regarding the dimensions and materials of
source 24 without departing from the scope of Applicant's
invention.
[0046] Beam source 24, and the plasma 9 generated therewith, have
many useful properties, including the following measured values
using the source described immediately above:
[0047] Plasma 9 is very dense, with ion densities exceeding
10.sup.12 per cm.sup.3 when using a DC power supply output of 1 kW
at 300 V. The ion saturation current was measured at over 100 mA
for the source dimensions given and these power supply settings.
The current probe surface was positioned 5 cm beyond the end of
nozzle 6 blocking plasma 9. Electron current with the probe
grounded is greater than 1 A.
[0048] Plasma 9 is uniform over the length of the source, minus end
effects at the turnarounds. This is important for applications
where uniformity of deposition, treatment, or etching is required,
as it is in most applications. Substrate widths of 3 meters or
greater can be uniformly processed. In operation, plasma 9 appears
as one cm wide uniform beam extending outwardly from nozzle 6.
[0049] Plasma beam source ("PBS") 24 is not a sputter source.
Rather, source 24 is useful for PECVD, plasma treatment, or etching
processes. Although sputtering of the liner material does occur,
only minimal amounts of sputtered material exit nozzle 6 for
several reasons. First, the magnetron plasma region 8 (FIG. 1) is
located deep inside the source. Sputtered liner material redeposits
on the liner, the shunts 10 and 11 and/or on the cover 5 and nozzle
6. Because the sputtered material readily condenses upon contact
with a surface, source 24 includes a `torturous path` for sputtered
material to exit the source. Second, by feeding process gas into
the discharge chamber above magnetron plasma 8, the flow of supply
gas to the plasma 8 is directed away from nozzle 6, creating
directional momentum effects opposing condensate flow out of nozzle
6.
[0050] A low sputter rate of the source is actually observed in
operation. For example, when depositing a PECVD Silicon Oxide
coating, after several microns of coating were deposited, the
resulting coating appeared optically clear. This clear coating was
produced using a copper liner 16. As those skilled in the art will
appreciate, sputtered copper in a mixture of oxygen and argon gases
comprises a black coating. No such black coating was observed
forming a silicon oxide coating on a substrate using source 24.
[0051] Pure reactive gas can be `burned` in source 24. Prior art
high density plasma sources implement filaments, low work function
materials, or field effect devices, to generate electrons. These
sources typically feed an inert gas, such as argon, into the
source. Use of a reactive gas such as oxygen inside the source
tends to greatly shorten electron source lifetimes. To accomplish a
reactive process, these sources feed oxygen into the plasma outside
the source, reacting a portion of the oxygen with the argon plasma
emanating from the source. While the efficiency of such prior art
sources is low, those sources are nevertheless used today for many
processes because no alternative exists.
[0052] In marked contrast, however, Applicant's beam source 24 the
production of a high density, pure oxygen plasma. This has
advantages to several processes. In addition, the vacuum pumping
requirements are also reduced because argon flow requirements are
not a factor when using source 24.
[0053] Applicant's beam source 24 can be operated over a wide range
of process pressures. As is typical for magnetron type sources, the
PBS can readily operate at pressures in the 1-100 mTorr region. In
addition to this pressure range, operation can be extended down to
the 10.sup.-5 Torr range used in evaporation processes. Such
pressures may be used because nozzle 6 limits gas conductance out
of the source. By feeding the process gas 27 into discharge cavity
26, the pressure inside discharge cavity 26 can be sustained in the
mTorr region, while outside the source the process chamber may be
maintained at a much lower pressure. Also, process gas flow
requirements are minimized because discharge cavity 26 can be
maintained in the required mTorr region with less gas 27 flow due
to the conductance limitation presented by the narrow nozzle 6
opening.
[0054] Plasma beam 9 extends outwardly for 100's of mm from nozzle
6 depending upon the free mean path inside the process chamber. At
3 mTorr for instance, the beam extends at least 300 mm outwardly
from nozzle 6. Formation of such a plasma beam allows beam source
24 to excel at many applications. For instance, non-planar
substrates can be uniformly PECVD coated, treated, etc.
[0055] Substrate 23 can be electrically isolated from beam source
24. Because the substrate is not part of the electrical circuit,
the substrate can remain floating or be separately biased by a
different power supply. In certain embodiments, beam source 24
comprises a standard magnetron power supply using variety of
frequencies, including. DC, or AC from 0-100 MHz frequencies.
Special high voltage power supplies or RF supplies do not have to
be used. The connection to chamber ground can also be made to
either side of the power supply. In FIGS. 1-3, box 3 and cover 5
are connected to ground. This is convenient for safety
considerations.
[0056] FIG. 4 shows beam source 24 in a PECVD coating application.
A mixture of argon and oxygen 41 are delivered to source port 4 via
tube 40. A monomer gas 43 is released outside the source. A
polymeric coating is deposited onto substrate 23 by polymerization
of monomer gas activated by the ionized gas in plasma 9. Because of
the conductance limitation of nozzle 6, and because of the high
density and directionality of the plasma 9 exiting through nozzle
6, the monomer gas 43 does not enter source 24. This is actually
observed when, after a coating run, the discharge cavity 26 of beam
source 24 is essentially free of PECVD coating.
[0057] The substrate 23 can comprise a multitude of materials and
shapes. Such substrates include, for example and without
limitation, flexible webs, flat glass, three-dimensional shapes,
metals, silicon wafers, and the like. Other physical and process
configurations are possible using beam source 24. For example, one
or more monomer gases can be introduced into the discharge cavity
26 without immediate buildup problems. In addition, certain monomer
gases, such as hydrocarbons, can be fed into the source for
extended periods. Beam source 24 may also perform other plasma
processes such as plasma treatment, surface cleaning, or reactive
ion etching.
[0058] FIG. 5 shows beam source 24 used to react evaporant 29 in an
electron beam evaporation web coating application. Drum 25 carries
web 23 over the deposition region. Crucible 27 contains evaporant
material 28. Electron beam source 26 projects beam 31 into crucible
27. Plasma 9 is directed into the evaporant cloud 29 to promote
reaction with the ionized gas of the plasma 9. Shield 30 limits the
interaction of plasma 9 with the electron beam 31.
[0059] Using prior art methods, complicated hollow cathode sources
have been used to accomplish evaporant reactance. Hollow cathodes
are inherently non-uniform as the plasma outside of the hollow
cathode is only diffusion limited. With Applicant's beam source 24,
the magnetic field lines 19 contain the electrons, and by
electrostatic forces, the ions are likewise contained in plasma
region 9. Also as described above, beam source plasma 9 is uniform
over the substrate width due to the closed drift nature of the
electron containment.
[0060] FIG. 6 depicts the beam source 24 applied to a planetary box
coater application. In this view the source 24 is shown along its
length rather than from an end view. In the illustrated embodiment
of FIG. 6, plasma beam 9 appears as a sheet of plasma. Source 24 is
positioned sufficiently remotely from the substrate supporting
planetary, at the bottom of the box coater for example, to allow
room for other deposition sources, such as electron beam, or
thermal evaporation sources, for instance. By combining the beam
source 24 with other deposition sources, coatings can be densified
by the action of plasma 9. In certain embodiments, pure argon is
used to densify a metal coating. In other embodiments, a reactive
gas is added to the argon.
[0061] A major advantage of Applicant's source over the prior art
is the ability to directly consume reactive gases, such as oxygen,
in the source. Prior art sources, due to the need for filaments or
other electron generation means sensitive to consumption by a
reactive gas, required the use of an inert gas in the source. In
these prior art sources, the reactive gas was fed into the process
chamber external to the source. The necessarily poor efficiency of
ionizing the reactive gas in the chamber rather than in the source
itself requires high source powers and high argon flow rates. In
contrast, using beam source 24 to produce a pure reactive plasma,
or a combination of inert and reactive as required, process
efficiency is increased and the overall pumping speed needed to
maintain the process at the correct pressure is reduced. As those
skilled in the art will appreciate, excess argon need not be pumped
away.
[0062] FIG. 7 shows beam source 24 disposed above a substrate 23,
such as a silicon wafer. In the illustrated embodiment of FIG. 7,
the stage 51 supporting the wafer 23 is translated, i.e. moved, in
the X and/or Y directions to uniformly treat wafer 23 with plasma
9. In FIG. 7 illustrated the ability to separately bias substrate
23 and source 24. Bias supply 52, in this case an AC supply of
sufficient frequency to pass current through the wafer 23, is
connected to stage 51. Beam source supply 17 produces plasma 9.
Without the bias supply 52, the insulating substrate 23 would
normally rise to the characteristic floating voltage of plasma 9,
i.e. typically between about -10 to about -70 volts for the beam
source 24 depending upon process conditions. By turning on bias
supply 52, the voltage drop across the plasma dark space between
the plasma 9 and substrate 23 can be changed, positively or
negatively, to a level required for the process. Because the
substrate 23 is not an electrode in beam source 24, it can be
separately biased.
[0063] FIG. 8 shows two beam sources, 24a and 24b, used to a
generate a large area uniform plasma 91 over a substrate. In the
illustrated embodiment of FIG. 8, the substrate comprises flexible
web 23 drawn over roll 64. The two beam sources 24a and 24b are
identical, except magnets 60 and 61 of source 24a, and the end
magnets (not shown) in source 24a are disposed such that the south
pole has a facing relationship with plasma 91, while source 24b has
magnet 62 and 63 north poles facing inwardly. This configuration
creates a sharing of magnetic fields between the sources and
produces the closed plasma region 91 as shown.
[0064] FIG. 9 shows a section view of source 90 configured for a
space propulsion application. The basic components of the magnetron
electron source and cusp magnetic field are the same as in earlier
figures. In source 90, the magnetic cusp fields 18 and 19 are
created by annular electromagnets 70 and 71. The electron source
magnetron plasma 8 is created with the liner tube 16. Liner 16 is
electrically isolated from box 3 by insulator plate 72 and from
electromagnet 71 by insulator ring 73. The propellant gas 27 is
introduced into gas cavity 79 through port 92. Gas 27 then flows
into discharge cavity 26 via gap 78 between liner 16 and opposed
electrode 5.
[0065] Cover electrode 5 is electrically isolated from round box 3
by insulator plate 76. Cover 5 has a nozzle portion 6 that fits
down into the annular opening in electromagnet 70. Liner 16 and
cover 5 are connected across power supply 74. The illustrated
embodiment of FIG. 11 includes a DC supply with the cathode
terminal connected to liner 16. In other embodiments, an AC or RF
power supply is used. Box 3 is connect to ground. When power supply
74 is turned on, and gas 27 is flowing into discharge cavity 26,
electrons created by magnetron plasma 8 are trapped in mirror field
region of magnetic field 18, and plasmas 9 and 39 are created.
Thrust is generated as the plasma 9 is expelled through nozzle 6.
One component of the thrust is generated by the magnetic nozzle
effect. After passing through magnetic mirror 39, electrons then
experience a decrease in magnetic field strength as they move
outwardly from nozzle 6. In response to this negative gradient
field, electron motion is converted from thermal spinning to
kinetic motion along the axis of the field lines.
[0066] The electrons in turn electrostatically pull ions into
accelerating away from the source. Another form of ion thrust is
produced if the magnetic field in region 18 is increased to confine
ions, i.e. to a magnetic field strength exceeding at least 1000
Gauss. Under this condition, ions are magnetically confined and
heated by the radial electric field as they pass through nozzle 6.
As those electrons exit the nozzle, they are accelerated by both
the electrostatic repulsion from anode 5 and by the magnetic nozzle
effect.
[0067] The electron confinement achieved using Applicant's source
includes physically limiting two of the possible three axial
magnetic field electron escape paths by liner 16. The three axial
magnetic field regions include: (i) cone shaped compressed region
18, (ii) cone-shaped compressed region 19, and (iii) planar disk
compressed region 170. When liner 16 is connected as the cathode of
a DC circuit, or is on a negative AC cycle of an AC power supply,
electrons are electrostatically reflected from the liner's
surfaces. As electrons attempt to reach the anode electrode 5, they
travel by collisional diffusion across field lines 19 and through
mirror region 39 to exit the source through nozzle 6 before
returning to cover 5. While diffusing across magnetic field lines,
the electrons also spiral along these field lines. By configuring
the source so magnetic field lines 170 pass through liner 16,
electrons moving along these field lines remain electrostatically
contained. If field lines 170 were allowed to pass through an
electrically floating surface or opposed electrode 5, some number
of electrons would escape through the compressed mirror of field
lines 170. Allowing only one axial magnetic field region 18 to be
open to electron escape increases the efficient use of electrons in
creating and sustaining plasma plume 9.
[0068] FIG. 10 shows beam source 100. In various embodiments,
source 100 is circular, annular, or extended length wise. In the
illustrated embodiment of FIG. 10, source 100 includes rare earth
magnets 1 and 2, and two power supplies 83 and 84. Power supply 83
connects cathode liner 16 to box 3. Insulator 81 separate box 3
electrically from cover 5. Power supply 84 connects anode cover 5
to box 3. Box three is grounded.
[0069] Using the illustrated configuration of FIG. 10, the plasma
potential can be adjusted relative to ground. This is useful when
applying the plasma 9 to a grounded substrate. By increasing the
plasma potential, the ion energy striking the substrate is
increased. FIG. 10 further illustrates process gas manifolds 80
built into cover 5. Small distribution holes 85 conduct the gas 27
uniformly along the length of source 100 into discharge cavity 26.
Facing the magnets 1 and 2 toward each other in a cusp arrangement,
creates a strong mirror compression ratio in mirror region 39. With
rare earth magnets 1 and 2, the field strength at the mirror apex
can exceed 500 Gauss. As electrons pass through this mirror region
39, they experience this strong field and their Larmor gyro radius
is correspondingly small. Under these conditions, when the plasma
is viewed from the end as in this section view, the plasma 9 width
passing through nozzle 6 is very narrow, on the order of 3 mm.
[0070] This is an advantage over vertically directed magnets of
Window and Savvides, Helmer, and others. A vertical magnet
orientation is shown in another preferred embodiment in FIG. 11.
With vertically oriented magnets, while a null region 25 is created
above the magnetron confined region. 8, the field strength is
typically less than 100 Gauss and the electron Larmor gyro radius
is larger. In the illustrated embodiment of FIG. 10, shunt 10 is
fitted into aluminum body 12. Shunt 10 reduces the sputter rate of
liner 16, and evens out liner 16 sputtering to make the liner 16
last longer. While helpful in this regard, shunt 10 is not
necessary to the fundamental source operation.
[0071] Body 12 is water cooled by extruded holes 82. Insulators 14
and 86 support cathode body 12 in box 3 and electrically isolate
the cathode, i.e. body 12 and liner 16, from box 3. Source 100 may
be rectangular having an extended length. End magnets, used to make
both magnetic field regions 8 and 9 closed paths, are not shown in
FIG. 10.
[0072] FIG. 11 shows beam source 1100 having vertically oriented
magnets. This magnet configuration is representative of a Type II
unbalanced magnetron magnetic field as taught by Window and
Harding. A range of magnet 97 shapes, and discharge cavity 26
shapes, can be implemented within the scope of Applicant's
invention. In the illustrated embodiment of FIG. 11, magnets 97
create two confinement regions: magnetron confinement 95 at cathode
98 surface 105, and mirror/nozzle confinement 93 through nozzle
104.
[0073] As in other embodiments of Applicant's source, a magnetron
electron generation region 101 is contained in a discharge cavity
103. The discharge cavity contains a null magnetic field region 95.
A aperture 104 in cover plate 91 has a centerline coincident with
the axis of mirror field 93.
[0074] Planar liner 98 is water cooled via gun drilled hole 99 and
is fitted into shunt 96. Magnets 97 and angled shunts 109, along
with shunt 96 produce the unbalanced magnetic field depicted.
Planar cathode 98 and magnet components 96, 97 and 109 are
suspended by electrical insulators (not shown) in electrically
floating box 90. Electrically floating cover plate 91 is fastened
to box 90. Cover plate 91 is water cooled via holes 92. Piping to
direct water to the cover plate 91 and cathode 98 is not shown. Gas
27 is piped into box 90 through threaded hole 100. Gas 27 flows
around magnet shunt 96 and into discharge cavity 103.
[0075] When power supply 108 is turned on, a magnetron plasma 102
lights and supplies electrons to mirror confinement region 106.
Electrons caught in mirror confinement region 106 collide with gas
27 also attempting to exit through the nozzle 104 opening, and
dense plasma 94 is created. The illustrated embodiment of FIG. 11
includes a separate anode 107. Cover 91 is not connected as an
electrode in the electrical circuit. Cover 91 comprises a
conductance limitation to the exiting gas 27, forcing the gas to
exit through the mirror confinement region 106 in nozzle 104. Given
the high mobility of electrons, positioning the return electrode
107 external to the source produces little noticeable change in
source performance after the source lights. Because the anode 107
is more distant from the cathode 98, a pressure spike may be needed
in cavity 103, depending upon the base pressure and the ignition
voltage of the power supply 108 used, to ignite the plasma 102.
[0076] Once a conductive plasma 102 has ignited, the anode
electrode can be located in any location within the process
chamber. When the anode electrode is the nozzle 104, some ion
acceleration benefits can be obtained as described earlier. In
illustrated embodiment of FIG. 11, the liner material comprises
aluminum. Aluminum is a good secondary electron emitter when oxygen
gas 27 is used. Moreover, the reactive product, alumina, formed on
the cathode surface 105 sputters very slowly. These are advantages
to beam source operation because a high electron current for a
given power is generated and the cathode material 98 is slow to be
sputtered away.
[0077] Other materials having these properties may also be used.
For example, when an argon plasma 94 is desired, carbon is a good
cathode material. While not an exceptional secondary electron
emitter, carbon sputters very slowly in argon. Note that FIG. 11 is
a section view. Source 1100 can be round, or rectangular, and can
be extended to lengths longer than 1 meter. The present invention
enables many applications and processes; Several have been
mentioned above. More will be apparent to those skilled in the art
While several embodiments have been presented, many others are
possible within the scope of the present invention.
* * * * *